Using camshaft adjusting devices, the fuel demand of an internal combustion engine is adapted to the power which it is to output in each case. Here, the lift of the intake valves is adjusted such that the fuel quantity required for the power which is to be output is injected into the combustion chamber. In order to vary the valve lift, the camshaft is rotated relative to the crankshaft. For this purpose, an adjuster is provided which is comparable to a hydraulic cylinder. The hydraulic oil of the internal combustion engine serves as a pressure medium. The lubricating oil circuit of the internal combustion engine is utilized as a pressure oil source. The adjuster is actuated by means of the electromagnetic hydraulic valve.
DE 198 53 670 A1 discloses a generic electromagnetic hydraulic valve which is designed as a proportional valve for controlling a device for the rotational angle adjustment of a camshaft relative to a crankshaft of an internal combustion engine and is composed substantially of an electromagnet and of a hydraulic valve part which is fastened thereto. Here, the electromagnet has a hollow-cylindrical magnet housing in which are arranged at least one coil winding and an axially moveable magnet armature. Also arranged in the magnet housing are a first and a second pole shoe. The valve part of the proportional valve is, in contrast, formed by a hollow-cylindrical valve housing which is open at the end side and in the hollow cylinder of which is accommodated a control piston which can be moved axially by the magnet armature of the electromagnet via a plunger rod. Furthermore, the valve housing has, at its outer periphery, a plurality of encircling annular grooves which are spaced apart from one another axially and into which are formed in each case a plurality of radial openings which open out into the hollow cylinder of the valve housing and are formed as transverse bores. The control piston, which is of hollow-cylindrical design, of the valve part is connected here at one end side to the plunger rod of the magnet armature and at the other end side to a pressure spring which acts counter to the force of the magnet armature, and said control piston has, at its outer periphery, two annular control sections which, depending on the current supply to the electromagnet, overlap or underlap the radial openings of at least two annular grooves in the valve housing. Furthermore, the control piston has, axially, a cavity which is embodied as a longitudinal bore, which cavity is open at the pressure-spring side and, at the plunger-rod side, is connected by means of a plurality of radial windows, which are formed as radial bores, to the hollow cylinder of the valve housing, and which cavity is provided for further conveying the hydraulic medium within the valve housing.
Further electromagnetic hydraulic valves of generic design are described in DE 197 45 411 A1, DE 199 38 884 A1, DE 199 56 160 A1, DE 100 56 200 A1, DE 102 39 207 A1 and DE 103 00 974 A1.
Such control pistons which are designed as hollow pistons are generally produced from an iron material. Since considerable friction is generated in the hydraulic valve between the plunger rod and the control piston, the latter must undergo a hardening process, for example a case hardening process. In order to prevent dimensional distortion of the control piston during said hardening process, which would adversely affect the operation of the valve, only that end face of the piston which bears against the plunger rod is hardened. This is carried out in such a way that the rest of the control piston is held by a copper sleeve which must additionally be cooled. After the hardening process, the entire control piston is washed and that end face which faces toward the plunger rod subsequently additionally undergoes a grinding process. It is obvious that such a production method of a control piston is very complex and therefore also expensive.